I just added 4GB of RAM so now have 8. I purchased the Lenovo 4GB through NCIX in Canada for $38.94CAD ($44 taxes in). Part No. 55Y3711 made in Korea (with 55Y3717 stamped on the actual stick). I do photo editing and the 8GB is working fine! The 4GB that had shipped with the X220 (that I got earlier...

Here is another discussion that supports that 16gb of RAM is possible:

http://forum.thinkpads.com/viewtopic.ph ... it=maximum

Ordered an X220